From disclosure DE 42 22 190 A1 is known such a radio telephone comprising an antenna switch for alternately operating in the transmitting and receiving states. For adjustment purposes is then arranged a .lambda./4 waveguide between transmitter output stage (31) and antenna (1). As a result, the special transformation properties for the initial and terminal resistance Z.sub.A and Z.sub.E respectively, of the .lambda./4 waveguide can be used. The terminal resistance Z.sub.E of a .lambda./4 waveguide is transformed on the input of the .lambda./4 waveguide in dependence on its wave resistance Z. By means of the parameters of the waveguide range such as geometric dimensions and material constants (.epsilon.,.mu.), the wave resistance Z can be set for the wavelength .lambda. to be used. The transformation of the resistance is generally effected in accordance with the relation Z.sub.A =Z.sup.2 /Z.sub.E. In the transmitting state, the .lambda./4 waveguide (32) connected to the transmitter output stage (31) transforms the antenna impedance in accordance with the requirements of the transmitter output stage (31). In this manner, the transmitter output stage (31) in the transmitting state is adjusted to match with the antenna (1) and the output power of the transmitter output stage (31) is fully passed on to the antenna (1). As a result of, inter alia, possibilities to change the wave resistance Z, a suitable transformation factor may be set. The second .lambda./4 waveguide (32) may be discarded for the transformation of the resistance when the wave resistance of the waveguide is set equal to the antenna resistance (see above, transformation equation).
The load impedance of a power amplifier in a transmitter output stage is adjusted to match with the input of an antenna to achieve a maximum efficiency of the transmitter output stage (ratio of the power fed to the antenna to the overall power used). An adjustment by means of a state-of-the-art .lambda./4 waveguide, however, provides a good efficiency (fixed transformation factor) only for an operating state with a certain output power P.sub.A and supply power U.sub.B of the power amplifier. In mobile radio telephones, for example, for mobile radio in accordance with the GSM standard, however, various levels are determined for the output power, which are to be exactly set by means of a power control in dependence on the transmission properties. Furthermore, the supply voltage U.sub.B varies in accordance with a discharge curve for the battery used. Consequently, as against the adjusted operating state, for example, with an increased supply voltage U.sub.B and regulated output power P.sub.A, the efficiency drops because the voltage U.sub.A on the output of the transmitter output stage remains constant. This also provides that the current I.sub.A on the output retains the regulated constant value, so that the efficiency diminishes strongly. All in all, often only a reduced efficiency of the transmitter output stage is achieved. This is a considerable disadvantage, especially for battery-operated mobile radio telephones because of the consequent shortened service life.